A prior art semiconductive device 12 is shown as part of a semiconductor wafer fragment 10 in FIG. 1. Device 12 is a field effect transistor formed proximate a semiconductive substrate 14. To aid in interpretation of the claims that follow, the term "semiconductive substrate" is defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term "substrate" refers to any supporting structure, including, but not limited to, the semiconductive substrates described above.
Field effect transistor 12 comprises a gate stack 16 comprising a gate oxide layer 18, a polysilicon layer 20, a silicide layer 22 and an insulative capping layer 24. Insulative capping layer 24 can comprise, for example, silicon nitride or silicon oxide. Silicide layer 22 can comprise, for example, tungsten silicide. Polysilicon layer 20 typically comprises conductively doped polysilicon, and gate oxide layer 18 comprises an insulative material, such as silicon oxide.
Spacers 26 are formed adjacent gate stack 16. Spacers 26 typically comprise an insulative material, such as silicon nitride or silicon dioxide.
Source/drain regions 28 are formed within substrate 14, and laterally offset from gate stack 16 by about a thickness of spacers 26. The formation of source/drain regions 28 typically comprises implanting a conductivity-enhancing dopant into substrate 14. Such implanting occurs after formation of spacers 26 to accomplish the shown lateral displacement of source/drain regions 28 from gate stack 16.
The dopant type within source/drain regions 28 and substrate 14 will vary depending on whether field effect transistor 12 is an n-type metal-oxide semiconductor (NMOS) or a p-type metal-oxide semiconductor (PMOS) transistor. For an NMOS transistor, source/drain regions 28 will predominately comprise n-type conductivity enhancing dopant, and substrate 14 will have a light background p-type dopant concentration. In contrast, if field effect transistor 12 is a PMOS transistor, source/drain regions 28 will predominately comprise p-type conductivity-enhancing dopant, and substrate 14 will have a light background n-type dopant concentration.
Lightly doped diffusion regions (LDD regions) 29 are formed beneath spacers 26 and constitute a part of the diffusion region. LDD regions 29 comprising a same dopant type as source/drain regions 28. LDD regions 29 are typically formed by implanting a dopant into substrate 14 after forming gate stack 16 and before forming spacers 26. Halo regions (not shown) can also be formed as part of the diffusion region. The halo regions will comprise an opposite type dopant as source/drain regions 28, and will typically be formed at junctions between regions 28 and 29, and substrate 14.
A continuing goal in semiconductor device fabrication is to minimize the device size. As field effect transistors become increasingly smaller, they become increasingly susceptible to short-channel effects. One way of reducing short-channel effects is to reduce a vertical depth of source/drain regions 28. In other words, to form shallow source/drain regions (i.e., source/drain regions that have a lowermost junction boundary that is less than 0.2 microns deep).
Forming shallow implants can be difficult. Generally, shallow implants cannot be formed simply by lowering implant energy, because such lower implant energy results in decreased focus of the implant, and corresponding loss of implant control. Accordingly, complex methods have been developed for forming shallow implants. In one method, a dopant is implanted into a first layer, such as a silicide, and then out-diffused from the first layer into a substrate to form a shallow implant. In another method a conductivity-enhancing dopant is implanted through an inorganic layer, such as silicon dioxide, and into an underlying substrate to form a shallow implant within the substrate. The inorganic layer must generally be removed from over the substrate after the implanting to enable further processing. Removal of the inorganic layers from semiconductive substrates is difficult.
For the above-discussed reasons, it is desirable to develop alternative methods for forming shallow implant regions in semiconductive substrates.